In OFDM transmission systems, the transmission band is subdivided into a number kmax of subcarriers, with each subcarrier being occupied individually with modulation, in general either PSK (phase shift keying) or QAM (quadrature amplitude modulation). An OFDM symbol is obtained as the resultant vector of the modulation on the individual subcarriers. In the transmitter, the OFDM symbol (which is effectively in the frequency domain) is transformed by means of a fast inverse fourier transformation, also referred to as IFFT, based on the kmax subcarriers in the time domain. In the receiver, kmax or more time signal sample values of a received OFDM symbol are transformed to the frequency domain again by means of a fast fourier transform, also referred to as an FFT, and are demodulated there (PSK or QAM demodulation).
One known OFDM transmission standard is the WLAN Standard IEEE-802.11a. In this Standard, an OFDM symbol is divided into kmax=52 subcarriers, with 48 fixed subcarriers being used for transmission of data information, and 4 fixed subcarriers being used for transmission of pilot information. An OFDM symbol in this case comprises 48 QAM data symbols and 4 PSK pilot symbols. Data transmission rates of up to 54 Mb/s are achieved using the WLAN Standard IEEE-802.11a.
A fundamental distinction is drawn between two different types of data communication for the data communication that occurs in a WLAN:
1. Sporadic Data Communication
In the case of sporadic data communication, data is interchanged only occasionally. One typical example of sporadic data communication is communication using the Internet Protocol. In this case, the user acts or reacts at certain time intervals, so that data is interchanged only occasionally, rather than continuously. One typical feature of data communication such as this is a relatively short packet length. When the packet lengths are short, the packet synchronization, also referred to as burst synchronization, is problematic since only a short time period is available for synchronization. Aspect elements of burst synchronization are burst detection, gain setting, antenna switching for antenna diversity, rough frequency synchronization, frame synchronization, symbol synchronization and initial channel estimation. Burst synchronization is based on the evaluation of a packet preamble, which precedes the actual payload data in the packet. Since packet preambles which are as short as possible are used in order to ensure the maximum possible transmission efficiency for short packet lengths, the burst synchronization must be completed very quickly within the short packet preamble.
2. Continuous Data Communication, in Particular Streaming
In the case of continuous data communication, data is transmitted continuously from a data source to a data sink. One typical example of continuous data communication is the streaming of audio and/or video data transmitted for digital broadcast radio (DAB, DVB-T). In order to ensure high transmission efficiency when streaming audio and/or video data, the data packets which are used for continuous data communication are in general very long in comparison to those used for sporadic data communication. If the data packets are very long, this results in the requirement for both the frequency synchronization and the channel estimation to be continuously updated in order to prevent desynchronization of data communication during a long data packet. This is because the frequency drifts over time, and the channel parameters vary continuously owing to the time invariance of the channel. Continuous updating of the frequency synchronization and channel estimation during transmission of the payload data of a data packet is also referred to as frequency tracking or channel tracking.
FIG. 1 illustrates the format of the packet preamble for the WLAN Standard IEEE-802.11a that has already been mentioned above. The definition of the packet preamble is taken from the Standard document for IEEE-802.11a-1999, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: High-Speed Physical Layer in the 5 GHz Band”, Sep. 16, 1999.
The preamble comprises a first part 1, which can be subdivided into 10 periodic segments B, also referred to as B segments. The identical segments B are short OFDM training symbols, which relate to only 12 subcarriers of the total of 52 subcarriers. OFDM symbols such as these form the basis for a plurality of sample values, represented in the time domain (that is to say after the IFFT). Owing to the smaller number of subcarriers, the time duration of the short OFDM training symbols B at 0.8 μs is considerably shorter than the time duration of 4.0 s of an OFDM symbol covering all 52 subcarriers (the time duration of 4.0 μs itself includes a so-called guard time of 0.8 μs). The periodically repeating OFDM training symbols B are used for rough frequency synchronization, burst detection and reception amplifier gain setting.
Furthermore, the preamble comprises a second part 2, which includes two periodic segments C. These preamble segments, which are referred to as C segments, are long OFDM training symbols which use all of the 52 subcarriers and thus each have a time duration of 3.2 μs. The major purpose of the long OFDM training symbols is the initial estimation of the channel parameters.
In addition, a guard interval CP with a time duration of 1.6 μs is provided between the short OFDM training symbols B in the first part 1 of the preamble and the long OFDM training symbols C in the second part 2 of the preamble. After the long OFDM training symbols C, an OFDM preamble symbol signal is transmitted, which contains information about the data rate and the length of the subsequent data payload 3 in the packet.
Since the receiver is not ready to receive until after the duration of a number of B segments owing to the need to carry out burst detection and the need to set the gain, it does not know the precise position of a short OFDM training symbol B within the 10 B-segment long chain. The first part 1 of the preamble is therefore not suitable for frame synchronization. Instead of this, frame synchronization is carried out on the basis of the transition between the first part 1 of the preamble and the second part 2 of the preamble, which describes a uniquely defined time within the data packet. This type of frame synchronization has the disadvantage that it is carried out relatively late within the preamble. If the entire process of frame synchronization cannot be completed in its entirety within the second part 2 of the preamble, data reception of the first OFDM data symbols within the data payload 3 is adversely affected. For sporadic data communication with a relatively short packet length, this means that a relatively large proportion of the data payload 3 may be disturbed, so that it becomes necessary to re-transmit the relevant data packet. Furthermore, the frame synchronization is relatively inaccurate, since the autocorrelation function has a relatively flat profile.
The document “Improved Frame Synchronisation for Spontaneous Packet Transmission over Frequency-Selective Radio Channels”, by S. Fechtel et al., Proceedings IEEE Int. Conf. on Personal, Indoor and Mobile Radio Comm., PIMR 1994, pages 353 to 357, The Hague, Netherlands, 1994, discloses a preamble format which is based on periodic segments, similar to the B segments, but has improved frame synchronization characteristics. The preamble as described in the cited document is obtained from segment-by-segment multiplication of a fixed initial segment, for example the B segment, by a mathematical sign sequence, whose elements either have the value +1 or the value −1. This preamble has considerably better autocorrelation characteristics than a preamble composed of periodic identical segments. This allows rapid and precise frame synchronization.
As has already been explained above, only 4 subcarriers of the total of kmax=52 subcarriers in the WLAN Standard 802.11a are intended for transmission of continuous pilot information. FIG. 2 shows the occupancy of the 52 subcarriers with the subcarrier index from −26 to +26 (the subcarrier with the index 0 at 0 Hz is not used). The pilot signals P.sub.−21, P.sub.−7, P.sub.7 and P.sub.21 are transmitted on the subcarriers with the indices −21, 7, 7 and 21, respectively, with each pilot signal corresponding to one PSK pilot symbol in each time step.
If pilot signals which are already known in the receiver are used for synchronization or for tracking, this is referred to as data-aided (DA) synchronization or data-aided (DA) tracking. If the synchronization or the tracking is based on the evaluation of data signals which are initially unknown in the receiver, this is in contrast referred to as decision-directed (DD) synchronization or decision-directed (DD) tracking.
In the WLAN Standard IEEE-802.11a, the pilot information on the 4 subcarriers is sufficient for continuous frequency tracking, so that the frequency tracking can be carried out as a completely DA-based frequency tracking process. Owing to the fact that the 4 subcarriers of the pilot signals are relatively far apart from one another in the frequency domain, sufficiently accurate continuous DA-based estimation of the channel parameters (channel tracking) is not possible for all of the subcarriers of an OFDM data symbol on the basis of these pilot signals. For this reason, the channel tracking is implemented as DD-based channel tracking in IEEE-802.11a-conformal receivers, that is to say the data information is used rather than the pilot information for channel tracking. One disadvantage of a DD-based implementation is, however, that demodulated and decided QAM data symbols are initially modulated onto the individual subcarriers once again at the receiver end, and may also have to be coded again. This process must in this case be repeated in the course of the averaging process for a plurality of OFDM symbols. In this case, it can be stated that the delay related to this and the complexity of the implementation are related to the accuracy of the channel tracking, that is to say the better the channel tracking accuracy, the greater is the implementation complexity and the greater the time delay, as well. For continuous data communication with long packets, in which sufficiently accurately acting channel tracking is essential, this means that the channel tracking can be implemented only with a very high degree of implementation complexity with the aid of the DD-based approach. For more detailed understanding of DD-based channel tracking, reference should be made to the publication “Channel Tracking in Wireless OFDM Systems” by H. Schmidt et al., SCI 2001, Orlando, Fla., 2001.
Section 4.5.3 of the DVB Standard document ETSI EN 300744, “Digital Video Broadcasting (DVB): Framing Structure, Channel Coding and Modulation for Digital Terrestrial Television”, European Standard, V1.4.1, 2001, describes the provision of additional scattered pilot signals—which are also referred to as scattered pilots—on fixed subcarriers, in addition to continuous pilot signals, in which case the subcarrier of a scattered pilot such as this changes from one OFDM symbol to the next. Effectively, a pilot signal which is scattered in this way moves over time over a large number of different subcarriers. This measure allows the pilot signals to cover a large number of subcarriers without this noticeably reducing the useful data rate.
The definition of a new OFDM-based WLAN Standard for the next generation is currently being worked on in the WIGWAM (Wireless Gigabit with Advanced Multimedia Support) research project, which is based on the previous WLAN Standard IEEE-802.11a, but has a considerably higher data transmission rate than the previous Standard IEEE-802.11a. The aim in this case is to achieve data transmission rates in the order of magnitude of up to 1 Gb/s. Various frequency bands between 5 GHz and 60 GHz are provided for radio transmission. Each frequency band has a width of approximately 500 MHz and allows the transmission of a plurality of OFDM channels. Particularly powerful terminals could also bundle a plurality of OFDM channels, which would make it possible to even increase the data transmission rate to several Gb/s. In addition to the increased data rate, one significant feature of this future WLAN Standard is that the Standard offers an improvement over the already existing WLAN Standard both for sporadic data communication and for streaming of video and audio data.